This invention relates to a radio apparatus having a function which compensates for non-linear distortion of a transmission power amplifier.
A power amplifier for amplifying a linear modulated signal used in wireless communications is required to have an amplifier with excellent linearity in order to suppress deterioration of transmission characteristics caused by spectrum characteristics and signal distortion. On the other hand, it is required in almost all applications that an amplifier deliver a high power efficiently at all times. In general, linearity and efficiency of an amplifier are characteristics that run counter to each other and a variety of distortion compensation schemes have been proposed in order to reconcile the two.
The field of next-generation mobile telephone systems (IMT-2000, etc.) based upon W-CDMA is one in which the present invention is particularly useful. With W-CDMA, code division multiplexing is used in direct-sequence spread-spectrum modulation and multiple-access for signal modulation. The transmitted signal has a wider band and a higher dynamic range in comparison with the narrow-band modulation and time division multiplexing schemes used heretofore in existing second-generation mobile telephones (PDC), etc. Accordingly, a power amplifier used in a W-CDMA apparatus is required to exhibit better linearity and higher efficiency than in the past.
FIG. 29 is a block diagram illustrating an example of a radio apparatus according to the prior art. A transmit-signal generator 1 transmits a serial digital data sequence and a serial/parallel (S/P) converter 2 divides the digital data sequence alternately one bit at a time to convert the data to two sequences, namely an in-phase component signal (xe2x80x9cI signalxe2x80x9d: In-phase component) and a quadrature component signal (xe2x80x9cQ signalxe2x80x9d: Quadrature component). A DA converter 3 converts the I and Q signals to respective ones of analog baseband signals and inputs these to a quadrature modulator 4. The latter multiplies the input I and Q signals (the transmit baseband signals) by a reference carrier wave and a signal that has been phase-shifted relative to the reference carrier by 90xc2x0, respectively, and sums the results of multiplication to thereby perform quadrature modulation and output the modulated signal. A frequency converter 5 mixes the quadrature-modulated signal and a local oscillation signal to thereby effect a frequency conversion, and a transmission power amplifier 6 power-amplifies the carrier output from the frequency converter 5. The amplified signal is released into the atmosphere from an antenna 7.
In a transmitting apparatus of this kind, the input/output characteristic of the transmission power amplifier develops non-linearity, as indicated by the dashed line in FIG. 30(a). Owing to this non-linear characteristic, non-linear distortion occurs and the frequency spectrum in the vicinity of transmission frequency f0 develops rising side lobes as indicated by the dashed lines in FIG. 30(b). This leads to leakage and interference between neighboring channels. For this reason, various distortion compensating techniques have been proposed, one of which is a predistorter (an distortion compensating device). A predistorter adds a characteristic that is the inverse of the distortion of a transmission power amplifier onto an input signal in advance, whereby the transmission power amplifier outputs the desired distortion-free signal.
FIG. 31 is a block diagram of a radio apparatus having a non-linear distortion compensating function, which uses a digital Cartesian scheme, as a prior-art example of a predistorter. Digital data sent from the transmit-signal generator 1 is converted to two signal sequences, namely an I signal xcexdi and a Q signal xcexdq, in the S/P converter 2, and these signals enter a predistorter 8. The predistorter 8 reads distortion compensation values xcex94xcexdi(n), xcex94xcexdq(n), which correspond to the input baseband signals xcexdi, xcexdq, out of distortion compensation tables 8a, 8b, adds these compensation values to the signals xcexdi, xcexdq and inputs the results to the DA converter 3. The latter converts the entered I signal xcexdi and Q signal xcexdq to analog baseband signals and inputs these signals to the quadrature modulator 4. The latter multiplies the input I and Q signals by a reference carrier wave and a signal that has been phase-shifted relative to the reference carrier by 90xc2x0, respectively, and sums the results of multiplication to thereby perform quadrature modulation and output the modulated signal. The frequency converter 5 mixes the quadrature-modulated signal and a local oscillation signal to thereby effect a frequency conversion, and the transmission power amplifier 6 power-amplifies the carrier output from the frequency converter 5. The amplified signal is released into the atmosphere from the antenna 7. Part of the transmit signal is input to a frequency converter 10 via a directional coupler 9, whereby the signal undergoes a frequency conversion and is input to a quadrature detector 11.
The quadrature detector 11 multiplies the input signal by a reference carrier wave and a signal that has been phase-shifted relative to the reference carrier by 90xc2x0, reproduces baseband signals xcexdxe2x80x2i, xcexdxe2x80x2q on the transmitting side and applies these signals to an AD converter 12. The latter converts the applied I and Q signals to digital data and inputs the digital data to a distortion compensator 8. At this time a phase shifter 13 applies a phase adjustment in such a manner that the phases of the demodulated baseband signals xcexdixe2x80x2, xcexdqxe2x80x2 will coincide with the phases of the input signals xcexdi, xcexdq. The AD demodulator 12 applies an AD conversion to the demodulated baseband signals xcexdixe2x80x2, xcexdqxe2x80x2 obtained by quadrature detection and inputs the resulting signals to the predistorter 8. The latter compares the demodulated baseband signals xcexdixe2x80x2, xcexdqxe2x80x2 and the input baseband signals xcexdi, xcexdq, updates the compensation values in the distortion compensation tables 8a, 8b based upon errors between the signals and stores updated distortion compensation values xcex94xcexdi(n+1), xcex94xcexdq(n+1) in the memories 8a, 8b. The operation described above is subsequently repeated.
With the digital Cartesian scheme described above, predistortion is carried out by obtaining distortion of the transmission power amplifier as an error along each axis of a rectangular coordinate system and adding characteristics that are the inverse of these errors to the respective axial components.
FIG. 32 is a prior-art example of distortion compensation based upon a feed-forward (FF) scheme. With the FF scheme, part of a signal that has been amplified by a main amplifier (transmission power amplifier) 6 is branched by a directional coupler 9, and an arithmetic unit 15 calculates the difference between the branched part of the signal and a signal obtained by subjecting the input signal to a delay adjustment and level adjustment. The difference signal is a non-linear distortion component produced by the main amplifier 6. The difference signal is amplified by a linear auxiliary amplifier 16, and a combiner 18 combines, 180xc2x0 out of phase, the output of the auxiliary amplifier and a signal that is result of delaying the main amplifier output by a delay line 17. As a result, distortion compensation is achieved by canceling out the distortion components.
With the conventional predistorter, the signal that has undergone predistortion is required to have a wide dynamic range in comparison with the dynamic range of the original signal in order to compensate for amplitude distortion of the amplifier (the power transmission amplifier). This means that a higher bit precision is required for the DA converter that subjects the predistortion signal to a DA conversion. In the case of a power amplifier used in W-CDMA in particular, the original signal is a code-multiplexed signal whose amplitude exhibits a large fluctuation and, moreover, is a wide-band signal owing to direct-sequence spread-spectrum modulation. With the conventional predistortion scheme, therefore, the DA converter requires a high bit precision and, at the same time, a high conversion speed. If such requirements are not met, a problem that results is deterioration of the distortion compensation characteristic.
Further, in predistortion of a power amplifier used in multicarrier W-CDMA, in which multiple carriers undergo common amplification, the DA converter is required to have even higher speed and higher bit precision capabilities. When application to W-CDMA devices currently developed is considered, a problem encountered is that the performance of currently existing DA converters cannot satisfy the requirements of high speed and high bit precision.
The aforesaid problems arise not only with regard to DA converters but hold true also for AD converters that sample a feedback signal for the purpose of updating distortion compensation coefficients.
Further, in a radio apparatus having a predistorter that compensates for amplifier distortion as a function of input power, the quadrature modulator and quadrature demodulator are implemented by analog circuits. A problem encountered is that amplifier-distortion estimation error grows owing to imperfections with these quadrature modulator/demodulators and a satisfactory distortion compensation characteristic is not obtained.
With the FF scheme, a problem that arises is that efficiency of the overall distortion compensating device declines because it is necessary to use a low-efficiency auxiliary amplifier that requires a high degree of linearity and because the delay lines and coupler are lossy.
Accordingly, an object of the present invention is to provide a radio apparatus that makes it possible to compensate for distortion of a transmission power amplifier by predistortion even though a DA converter and an AD converter are not required to have high speed and a high bit precision.
Another object of the present invention is to provide a radio apparatus using a digital quadrature modulator and digital quadrature demodulator to eliminate the imperfections of analog quadrature modulator/demodulators, whereby a satisfactory distortion compensation characteristic is obtained.
Another object of the present invention is to provide a radio apparatus having a highly efficient distortion compensation device without use of an auxiliary amplifier or delay lines.
In a radio apparatus according to the present invention, a transmit signal (main signal) and a distortion component (error signal) added onto the main signal are each subjected to a DA conversion independently, after which the converted signals are combined and input to a transmission power amplifier. If this arrangement is adopted, the amplitude of the error signal will be small with respect to the amplitude of a predistortion signal obtained by adding a characteristic that is the inverse of amplitude distortion to the main signal. As a result, it is possible to lower the bit precision of a DA converter, which outputs only the error signal. Further, a DA converter that outputs only the main signal need not have a wide dynamic range and the bit precision of this DA converter an be lowered as well.
Further, in a case where compensation is applied to non-linear distortion of a transmission power amplifier that amplifies and transmits a multicarrier signal carrying a plurality of transmit signals, a signal obtained by DA-converting an error signal is combined with a frequency-multiplexed signal obtained by subjecting DA-converted signals of respective ones of transmit signals to a frequency-shift operation decided by carrier spacing and multiplexing the frequency-shifted signals, and the combined signal is input to the transmission power amplifier. If this arrangement is adopted, each of the transmit signals and the error signal are DA-converted independently and combined. As a result, neither of the DA converters need have a wide dynamic range and, hence, bit precision can be suppressed.
In a radio apparatus according to the present invention, predistortion processing, in which distortion compensation coefficients are read out of a distortion compensation coefficient table and a transmit signal is subjected to distortion compensation using the distortion compensation coefficients, and coefficient update processing, in which the distortion compensation coefficient table is updated using a feedback value (amplifier output signal) that has been sampled by an AD converter, are executed separately in terms of time. By thus executing the updating of distortion compensation coefficients and predistortion processing separately in terms of time, a real-time feedback loop is not formed. As a result, sampled values that are continuous in time are not required as values sampled by the AD converter, thus making it possible to mitigate the requirement that the AD converter have a high speed. Further, since distortion compensation is performed by predistortion processing, there is no need for an auxiliary amplifier and delay lines, which were an impediment to an improvement in efficiency with the FF scheme. This makes it possible to raise the overall efficiency the transmission power amplifier that undergoes distortion compensation.
Further, in a radio apparatus according to the present invention, a wide-band sample-and-hold circuit is provided on the input side of an AD converter that samples a feedback value. If such a wide-band sample-and-hold circuit is connected to the input of an AD converter and a wide-band signal whose spectrum has spread owing to non-linear distortion is sampled by this circuit, then it will be possible to perform an AD conversion at a sampling rate lower than the Nyquist rate. In other words, though it is necessary that a sampled band be widened enough (beyond the Nyquist rate) to enable observation of the distorted signal, the sampling rate (the number of samples per unit time) can be set independently of the Nyquist rate and the requirement that the AD converter have a high speed can be mitigated.
In a radio apparatus according to the present invention, a quadrature modulator and a quadrature demodulator are implemented by digital operations. Adopting a digital quadrature modulator/demodulator makes it possible to reduce the error of the quadrature modulator/demodulator to less than 1 LSB of DA and AD converters. This makes it possible to eliminate deterioration of distortion compensation characteristics caused by imperfections in a quadrature modulator/demodulator.